Current aircraft monitoring systems typically use aircraft communications addressing and reporting system (ACARS) data in combination with radar data in order to track the progress of aircraft. This data may be used by air traffic controllers or alternatively provided as a service to aircraft operators.
In the ACARS system, each aircraft is fitted with a VHF transceiver for providing a data link between the aircraft on-board equipment and ground equipment. This data link may be provided through a direct transmission from the aircraft to a ground station, or alternatively the aircraft may transmit the data to a satellite, which then forwards the data to a satellite ground station. These transmissions are received at the ground stations by a data link service provider that then routes the data to the air traffic controllers or aircraft operators.
The periodicity within which a given aircraft will emit ACARS data transmissions is configured by the operating airline and is typically in the order of ten to twenty minutes. This is generally determined in order to provide a balance between receiving up to date data and the per message costs associated with the data transfer. This periodicity is set by appropriately programming the on-board avionics during maintenance of the aircraft and cannot be changed during a flight.
In view of this relatively long period between consecutive ACARS message transmissions, significant distances can be covered by an aircraft between the transmissions, which can in turn lead to an uncertainty in the estimated position and path of an aircraft. Furthermore, the time stamp for any given ACARS transmission is only accurate to within a minute and the position data is reported within an accuracy of three decimal places.
If the aircraft is forced to circle in a given area of airspace, for example, in an airport holding pattern, this will not be immediately apparent from the ACARS data as the aircraft will likely have performed a full circle by the time a subsequent ACARS transmission is carried out. This can lead those monitoring the ACARS data to be unsure as to whether these data transmissions are erroneous or if the aircraft truly has remained in a given area of airspace between subsequent ACARS transmissions.
Increasing the standard frequency (i.e. reducing the period between consecutive transmissions) of ACARS messaging, as programmed into the aircraft's avionics during maintenance, would provide a more up to date set of position data. However, if each airline were to do this as a standard across the board then a large burden would be placed on the ACARS network, since it is a one-to-one digital data link system. This may overload the network and reduce its reliability and accuracy.
Airspace across the world is split up into a number of three-dimensional (3D) blocks of space known as sectors. Each sector has one or more air traffic controllers that communicate with and are responsible for the safety of aircraft operating in, or about to enter, that airspace sector. These controllers work for Air Navigation Service Providers (ANSPs) and are trained to manage the aircraft such that there is a safe and orderly flow of aircraft from point to point in the most efficient manner.
In order to achieve this, air traffic controllers communicate with aircraft to give active support and authorisations as well as to receive information from the aircraft. Usually this communication is carried out over voice radio such as radio transmissions in the VHF or HF bands. One issue with voice radio is that only one transmission can be made on a given frequency at a given time and so, even if there is a strong radio signal, transmissions may be cut off or become unintelligible. In order to ensure that transmissions are accurately received, it is necessary to read back the communication, which also increases the time it takes for a given communication to be completed.
Furthermore, voice communications can be subject to misunderstandings or language barriers, voice quality can be low, and VHF voice frequencies are subject to high traffic congestion. To combat these negative aspects, a committee was set up to establish a new system, the Future Air Navigation System (FANS), to improve these communications, for example by using a data link system to encapsulate messages between the ANSP and the aircraft.
A number of standard format communications have been determined than can be used to send common commands such as level or altitude assignments, crossing constraints, lateral deviations, route changes and clearances, speed assignments, radio frequency assignments, and various requests for other information, with the option of a free-text message for communications that fall outside of the standard list of common commands or responses.
These communications are commonly known as a Controller-Pilot Data Link Communications (CPDLC) and they eliminate the need to validate communications by multiple transmissions and reading back as both parties can see the communication in text form and the communications are available on demand such that they can be easily reviewed later or printed.
These data link messages are commonly encapsulated and transmitted using the Aircraft Communications Addressing and Reporting System (ACARS) protocol. Aircraft using ACARS may be fitted with a VHF and/or HF transponder for providing a data link between the aircraft on board equipment and ground station equipment. This data link may be provided through a direct transmission from the plane to the ground, or alternatively through a microwave transmission via a satellite. These transmissions are received at the ground by a data link service provider and then routed to aircraft operators by the data link service provider for a charge per message. Messages transmitted from the aircraft to a ground system may be referred to as downlink messages and messaged transmitted from a ground system to the aircraft may be referred to as uplink messages.
Another aspect of FANS is the ability to set up an Automatic Dependent Surveillance Contract (ADS-C). ADS-C uses the FANS avionics systems that are a part of the on-board Flight Management Systems (FMS) of FANS equipped aircraft to automatically provide information such as the aircraft's position, altitude, speed, intent and meteorological data to users such as ANSPs or airlines. At a minimum the ADS-C message will contain three-dimensional position information, the timestamp corresponding to the position information and a figure of merit (FOM) that indicates the accuracy of the position data.
The contract is defined by the ground system of the end user and may indicate that communications should be sent from the aircraft to the end user's ground systems in response to specified periodic, or demand based, or event based criteria or a combination of these criteria. Up to five separate ground systems are able to maintain ADS contracts with a given aircraft and, currently, these ADS-C connections are typically used by the air traffic controllers of ANSPs that have FANS enabled ground systems to reduce the reliance on voice channel dialogues between the pilot of an aircraft and the air traffic controller, which in turn reduces the workload of both the controller and the pilot and allows the separation between respective aircraft to be reduced.
The ANSP may determine the data link capabilities of a given aircraft by exchanging Air Traffic Service Facilities Notification (AFN) messages with the aircraft. These messages may also include the address information that allows a subsequent FANS session to take place.
In the past, airlines have relied on the reports issued by air traffic controllers and only used passive means for monitoring the status of their aircraft. It has been appreciated by the applicant that a proactive system for airlines to monitor the status of their aircraft that can be developed quickly and implemented using an aircraft's existing equipment is desirable.